The present invention relates to digital printing, and more particularly to field calibration of digital color printers. A large share of personal computers now sold include high resolution color displays for viewing color images. Using thermal inkjet technology, personal color printers are capable of approximating the color images viewed on such displays. By precisely controlling ink dot size and ink dot color placement, color halftone images may be printed. However, variability in inks, papers and drop volume cause problems in exactly reproducing desired shades and color tones. Similar problems occur in other printing schemes which use pigmented inks or pigmented dry toners, such as color laser printers. These variations in pixel color, density and dot size result in poor reproduction and color distortions.
Aside from ink and paper variations, the very nature of digital printing creates other problems. Because only a limited number of discrete dot sizes of a given color ink are used, some technique for tonal averaging or smoothing is required to yield proper tones for those "in between" colors not ordinarily printable. A system using such averaging techniques trades spatial resolution for tonal veracity. These techniques are generally called digital half toning methods. Following the algorithm of any of these methods, dots are distributed upon the paper in patterns or populations so as to create the visual impression of an intensity between that of white paper and full ink coverage. One such technique for smoothing color variations is the error diffusion method, where the difference between a desired color density at a pixel and its actual printed density, the error, is distributed to adjacent pixels. Floyd, R. W., and Steinberg, L., "An Adaptive Algorithm for Spatial Gray Scale, Pro. SAID, Vol. 17/2 (Second Quarter 1976), pp. 75-77. The distributed error is incorporated in calculating the neighboring pixel's printed dot size and then a similar process of diffusing any error in the next computation is undergone. However, to correctly implement error diffusion, one requires precise knowledge of available pixel densities. Other methods make use of groups of dots, for example Clustered Dot Dither or Super-pixel algorithms. Here the pattern of dot that most closely approximates the desired print density is chosen and no error processing occurs. For all these methods and approaches for approximating colors on paper, the applied color densities are influenced by the choice of ink and paper and vary with the tolerances and stability of the printing method.
Until now, one has designed personal color printers with some available set of dot sizes, calibrated the printer at a factory, and then relied upon the factory calibration despite changing ink/paper combinations used during the printer's life. A factory calibration process typically involves measuring print samples on a selected paper stock using relatively expensive laboratory equipment such as optical densitometers, colorimeters, and the like. Despite these centralized factory calibration procedures, there is no guarantee that a faithful reproduction will result when one uses somewhat different paper stock or a new batch of ink. Though it is conceivable that the printer could be recalibrated at the factory using a standard densitometer, it is not practical for users of personal color printers to return their printers to the factory for recalibration whenever paper stock or ink are changed. Similarly, though it is conceivable that the printer could be recalibrated in the field using the standard densitometer, standard densitometers are relatively expensive and require frequent maintenance, typical of laboratory equipment. It is therefore impractical to send such standard densitometers to users of personal color printers to recalibrate the printers in the field whenever paper stock or ink are changed.
FIG. 1 is a partial block diagram illustrating components and operation of a typical standard densitometer 100 used as a standard in measuring color density of an object sample under test. As discussed herein, the typical densitometer includes components that are constructed to conform to rigid uniform standards developed by such organizations as the American National Standard Institute (ANSI) and the International Standards Organization (ISO). Such construction greatly adds to the size, weight, maintenance, and cost of densitometer components and to overall size, weight, maintenance, and cost of the densitometer. For example, the typical densitometer includes a relatively expensive light source 104, which requires frequent replacement. In conformance with densitometer component standards, the light source is constructed to maintain long term stability of a particular spectral energy distribution. For example, the standards have previously been described in terms of a tungsten filament lamp providing an influx from the lamp operating at a Plankian spectral energy distribution of 3000 degrees Kelvin. An illustrative standard for the densitometer light source illuminant is known in the industry as 2856K ANSI. Because of the high temperature of operation, the 3000 degree tungsten filament lamp has a reduced Mean Time Between Failure (MTBF) and must be frequently replaced as part of a laboratory equipment maintenance schedule. While such maintenance requirements can be tolerated in equipment used in centralized facilities such as laboratories and factories, they would be quite inconvenient in equipment used by consumers for field calibration of color printers.
As shown in FIG. 1, the light source projects light through a precision ground glass lens 106 mounted in collimator housing 102, which serves to focus light from the light source into a narrow collimated beam of light rays 110. The light rays transmitted through the lens project through a precision machined aperture 108 in the collimator housing. Dimensions of the aperture determine the size of an irradiated area of the object sample under test. Various standards have been defined for preferable sizes of the irradiated area.
As the light rays are projected onto the object sample 112, electromagnetic radiation shown as light rays 114 will be reflected from the object sample under test 112. For purposes of detecting the reflected light rays 114, a rotatable spectral filter apparatus 116 is provided. The filter apparatus 116 can include a plurality of densitometer filters 118, 119, 120, and 122 which are employed for purposes of analyzing spectral response of the object sample under test. FIG. 1 shows a red densitometer filter 118, a neutral densitometer filter 119, a green densitometer filter 120 and a blue densitometer filter 122. The densitometer filters are relatively expensive, in part because they are constructed to have spectral transmittance characteristics in conformance to a particular densitometer filter standard. For example, an illustrative densitometer filter standard is known in the industry as the ANSI Status T Color response.
The spectral filter apparatus 116 shown in FIG. 1 includes not only the filters 118, 119, 120 and 122, but is also shown as including a shaft 124 having one end connected to a rotatable "wheel" 126 on which the spectral filters are positioned and spaced apart. The other end of the shaft 124 is connected to a manually rotatable knob 128. In the actual mechanical configuration of the densitometer 100, the knob 128 would be made accessible to the user for purposes of manual rotation of the wheel 126 so as to selectively position the individual filters as desired. In FIG. 1, the red filter 118 is shown as being appropriately positioned for transmitting a portion of the reflected light rays 114 therethrough.
Each of the filters can be individually selected by rotating the wheel to align the desired filter with a densitometer photo-electric sensing element 132. In general, the reflected light rays 114 are filtered as they pass through the selected filter to be received by a photo-electric sensing element 132. In response to the filtered light rays, the photo-electric sensing element produces an electrical current signal on line pair 134, which is coupled to analog electronic processing and adjustment circuits. The photo-electric sensing element is adapted so that magnitude of the electrical current signal is substantially proportional to the intensity of the light rays 130 sensed by the photo-electric sensing element 132.
The analog processing and adjustment circuits produce a color density value in response to the electrical current signal received. The color density value is then displayed on a display device coupled to the processing and adjustment circuits. For example, as shown in FIG. 1, an amplifier 136 produces an output voltage in response to receiving the electrical current signal on the line pair 134. A first calibration parameter is varied by adjusting gain of the amplifier 136, thereby varying magnitude of the output voltage on voltage signal line 138. Gain adjustment circuitry of the amplifier is representatively shown as a first adjustable resistance 139 in FIG. 1. The output voltage from the amplifier on line 138 is applied as an input signal to a logarithmic voltage converter 140. The logarithmic voltage converter is adapted to provide an output on an output line 142, which corresponds to the optical density measurement for the object sample and the particular configuration of the spectral filter arrangement 116. A second calibration parameter is varied by adjusting gain of the logarithmic voltage converter 140, thereby varying magnitude of the voltage on signal line 142. Gain adjustment circuitry of the logarithmic voltage converter is representatively shown as an adjustable resistance 144 in FIG. 1. A display device 146 is coupled to the output line 142 for displaying the optical density measurement.
Although the constituent components of densitometers are constructed to conform to rigid standards, minor calibration of the densitometer is needed periodically to compensate for small variations and drift of the constituent components. By performing small calibration adjustments, each densitometer can be made to produce color density measurements of the object sample under test that are in close agreement with those produced by every other densitometer. To correct for small variations and drift, the analog processing and adjustment circuits of the densitometer are typically constructed so that a few calibration parameters can be varied, thereby producing small calibration adjustments. For example, adjustment circuits 139, 144 of the amplifier and logarithmic converter shown in the densitometer of FIG. 1 provide for relatively small calibration adjustments by varying the first and second calibration parameter. Similarly, other standard densitometers provide additional small calibration adjustments by additionally varying a third and fourth calibration parameter. Calibration of standard densitometers is relatively simple because densitometer components are already constructed to conform to rigid uniform standards. However, as discussed previously, such construction greatly adds to the size, weight, maintenance, and cost of densitometer components and to overall size, weight, maintenance and cost of the densitometer.
The electronic printer industry has faced a great challenge to produce small, lightweight, reliable, inexpensive color printers capable of faithfully rendering color images, despite normal variations in paper stock and in ink color, dot size, and density. The development of an improved digital color printer, capable of self-calibration in the field and able to render correct color reproductions, would represent a major technological advance. The accurate and detailed color images that such a printer could produce would satisfy a long-felt need within the industry and offer to a large number of users a versatile, faithful and inexpensive color printing device.